Transducer for converting fluid pressure oscillations into mechanical oscillations

ABSTRACT

A transducer for converting fluid pressure oscillations into mechanical oscillations comprising a piston member in a cylinder, the piston member and the cylinder being relatively movable, first and second opposing faces of the piston member being respectively disposed in first and second chambers in the cylinder, each chamber being adapted to contain fluid under pressure, and means for connecting at least the said first chamber to a respective source of oscillating pressure fluid to cause relative oscillating movement of the piston member and the cylinder, excessive relative movement of the piston member and the cylinder in one sense being limited by a snubber, the piston and cylinder being predisposed to relative movement in the opposite sense to a predetermined position by means including limited fluid flow between the first and second chambers, valve means being provided to vent the fluid from the second chamber to drain when the piston and cylinder are in said predetermined position.

United States Patent Inventors Appl. No. Filed Patented Assignee Priority TRANSDUCER FOR CONVERTING FLUID PRESSURE OSCILLATIONS INTO MECHANICAL OSCILLATIONS 12 Claims, 2 Drawing Figs.

lnt.CI F0lb 7/18 Field of Search 92/81, 84, 85, 86,134,l43,142;91/232, 234, 235, 321, 408,

References Cited UNITED STATES PATENTS Primary ExaminerMartin P. Schwadron Assistant Examiner- Leslie J. Payne Attorney-Cushman, Darby and Cushman ABSTRACT: A transducer for converting fluid pressure oscillations into mechanical oscillations comprising a piston member in a cylinder, the piston member and the cylinder being relatively movable, first and second opposing faces of the piston member being respectively disposed in first and second chambers in the cylinder, each chamber being adapted to contain fluid under pressure, and means for connecting at least the said first chamber to a respective source of oscillating pressure fluid to cause relative oscillating movement of the piston member and the cylinder, excessive relative movement of the piston member and the cylinder in one sense being limited by a snubber, the piston and cylinder being predisposed to relative movement in the opposite sense to a predetermined position by means including limited fluid flow between the first and second chambers, valve means being provided to vent the fluid from the second chamber to drain when the piston and cylinder are in said predetermined position.

TRANSDUCER FOR CONVERTING FLUID PRESSURE OSCILLATIONS INTO MECHANICAL OSCILLATIONS This invention relates to a transducer for converting fluid pressure oscillations into mechanical oscillations.

The invention provides a transducer for converting fluid pressure oscillations, comprising a cylinder, a piston member in said cylinder, said piston member and said cylinder being adapted for relative movement, said cylinder having first and second chambers adapted to contain fluid under pressure, said piston member having first and second opposing faces disposed in said first and second chambers respectively, means adapted to connect at least said first chamber to respective source of oscillating pressure fluid to cause relative oscillating movement of said piston member and said cylinder, said cylinder including means in said second chamber defining a snubbing space adapted to contain fluid, said piston member including a portion adapted to enter said snubbing space and to compress fluid in said snubbing space upon excessive relative movement of said piston member and cylinder in one sense, means defining a restricted outlet from said snubbing space when said portion of said piston member has entered said snubbing space, means predisposing the piston member and cylinder to relative movement to a predetermined position in a sense opposite to said one sense; said means including means adapted to permit limited flow of fluid from said first chamber to said second chamber; valve means to vent said second chamber to a low pressure region upon relative movement of said piston member and cylinder in said opposite sense to said predetermined position.

The restricted outlet may be at least one narrow conduit extending from the snubbing space to the second chamber.

In one aspect of the invention, the or each narrow conduit may be a narrow groove.

. Upon initial said excessive relative movement, the at least one groove may communicate with the snubbing space or the second chamber over a substantial fraction of its length, further excessive relative movement in the said one sense reducing the said fraction whereby the resistance afforded by the at least one groove to the flow of fluid therethrough is increased.

The at least one narrow groove may be situated on a circumferential surface of thepiston member and may extend from the said second opposing face of the piston member towards the first opposing face for part of the axial length of the circumferential surface.

Preferably, the average'magnitude of the said restricted flow of fluid per cycle of oscillation of the piston member may be 5 percent to 20 percent, preferably 5 percent to percent, of the swept volume of the first chamber.

By average magnitude we mean the average magnitude per cycle taken over several cycles of oscillation of the piston member, and by swept volume we mean the effective area of the first face multiplied by the amplitude of the oscillating movement.

The said one chamber may be the second chamber, the first vent the means valve may comprise a port in the wall of the cylinder which is connected to the said low pressure region, said port being connected to the second chamber via said at least one groove upon the said relative movement to said second predetermined relative position.

When the piston member and the cylinder are in the said predetermined relative position, the at least one groove may only communicate with the said port at its end remote from the said second face, further relative movement in the said opposite sense causing the at least one groove to communicate with the said port at a part of its length nearer the said second face, whereby the resistance afforded by the at least one groove to the limited flow of fluid from the said one chamber is reduced.

The second vent means may comprise a duct through which the first chamber is in limited communication with the second chamber. The duct may contain a valve which when in a first position, allows a relatively unlimited flow of fluid to pass from the first chamber to the second chamber, and when in the second position allows relatively limited flow from the first chamber to the second chamber.

The invention also provides apparatus comprising a source of oscillating pressure fluid and a transducer as set forth above, a respective source of oscillating pressure fluid being connected to at least the said first chamber and the piston member being connected to a driven load.

There may be provided a further source of oscillating pressure fluid connected to the second chamber, the further source of pressure fluid being in antiphase relationship with the first mentioned source.

The invention will be described, merely by way of example, with reference to the accompanying drawings wherein FIGS. 1 and 2 show alternative embodiments of the invention.

The terms left" and right as used herein are to be understood as referring to the directions as seen in the drawings. Referring to FlG. l a transducer according to the present invention has a cylinder 10 with a piston member 12 mounted therein for relative sliding movement. The piston member 12 has a central relatively large diameter portion 14, and two outer portions 16 and 18 of somewhat smaller diameter. The diameter of the outer portion 18 is intermediate that of the portion l4, 16.

The end face 20 of the smallest diameter portion 16 constitutes a first operative face of the piston member, and a shoulder 22 formed by the change in diameter between the large diameter portion 14 and the intermediate diameter portion 18 of the piston constitutes a second operative face of the piston member. The faces 20 and 22 are parallel and oppositely directed, so that respective fluid pressure forces acting on the respective faces are in opposition.

Face 20 is disposed in a first chamber 24 of the cylinder 10, and face 22 is disposed in a second chamber 26 of the cylinder. Each chamber is adapted to contain fluid under pressure.

The first chamber is connected to a source of oscillating pressure fluid 28 via line 30. The oscillating pressure fluid will cause relative oscillating movement of the piston member 12 and the cylinder 10, the fluid in chamber 26 constituting a fluid spring. The volumes of the chambers 24 and 26 are chosen, having regard to the mass of oscillating parts of the transducer and the frequency of oscillation of the oscillating pressure fluid, so that the chamber 24 functions as a capacitor chamber, the pressure fluid oscillations therein being phaseadvanced relative to the oscillations in the chamber 26, resulting in the mechanical oscillations being in phase with the fluid pressure oscillations produced by the source 28. A minimum loss of energy is thus obtained.

In this exemplary embodiment of the invention, the transducer is embodied in a rock-drill, the cylinder 10 forming the body of the drill and being provided with handles (not shown) for ease of operation. The oscillations of the piston 12 are communicated to a bit 32 of the rock-drill, to produce useful work. The piston is not connected to the bit 32 but strikes it to produce an almost instantaneous force suitable for rockdrilling.

The transducer may alternatively be connected to, for example, a tine or group of tines of earth moving equipment, or moving parts of an agricultural implement, such as the blades of a grass-cutting machine.

It will be appreciated that the drawing is purely diagrammatic, and that in practice the cylinder 10 is made up of several parts to facilitate assembly. Consequently, in practice, there are several interfaces through which slight, but almost unavoidable, leakage from either of the chambers 24, 26 may occur. In particular, leakage may occur via bores 34, 36 in the walls of the chambers 24, 26 through which the piston member passes.

There is therefore provided in the bores 34, 36 respective annular leakage grooves 35, 37, 39 to receive fluid that may leak from the chambers 24, 26. The leakage grooves are respectively connected via lines 41, 42, 43 to a tank 40 which is at relatively low pressure. The leakage fluid is pumped back into the system via a makeup pump 44, a nonretum valve 46 and a connection 48 to line 30.

Disposed between the chambers 24, 26 of the cylinder is a further chamber 38 which is open to the atmosphere. This chamber receives any excess leakage that may pass the grooves 35 and 37, and also ensures that the face 21 between the portions 14 and 16 of the piston member 12 is not disposed in a closed volume. If this were so, that volume would act as a further fluid spring, and may reduce the efficiency of the transducer.

If during operation of the rock-drill the drill bit suddenly breaks in to a region of relatively soft material, or if the drill body is withdrawn quickly, excessive relative movement of the piston member 12 and the cylinder 10 may occur. That is to say, the piston member may move excessively far to the right, or the cylinder may move excessively far to the left. This is undesirable since in extreme cases the portion 14 of the piston member 12 may strike the end wall of chamber 26.

To deal with this problem, a wall 50 of the chamber 26 is provided with a counterbore 52, the counterbore 52 extending axially in a rightward direction and being the same diameter as the bore 36 within which the piston portion 14 moves, and coaxial therewith.

There is thus provided a snubbing space 54 defined by the counterbore 52 and the external surface of the piston portion 18.

The portion 14 of the piston member 12 has upon its circumferential surface at least one axially extending groove 56 which extends from the face 22 leftwards towards the face 20, there preferably being four such grooves 56 as shown. The grooves 56 do not extend for the whole length of the portion 14, but stop somewhat short of the left-hand end thereof.

Rightward movement of the piston member 12 will result in the right-hand end of the portion 14 entering the closed space 54, trapping fluid therein.

Communication with the snubbing space 54 is thus restricted to the flow area afforded by the narrow conduits constituted by the grooves 56, and further rightward movement of the piston member can only occur if some of the fluid trapped within the snubbing space 54 is forced out of the space via the grooves 56. The fluid in the space 54 must be compressed to overcome the resistance to flow of the grooves 56, and the increased pressure thus acts upon the face 22 of the piston to oppose further rightward movement thereof.

Thus relative movement of the piston member 12 and the cylinder 10 in one sense to a relative predetermined position causes communication with the snubbing space 54 to be restricted, further relative movement in the said one sense compressing the fluid in the said space and forcing it through the said restricted communication, the pressure of the fluid in the snubbing space 54 acting to oppose said further relative movement.

Whenthe piston portion 14 initially enters the space 54, the grooves 56 communicate with the chamber 26 over almost the whole of their length. The fluid escaping from the snubbing space 54 thus has only to flow through a relatively short portion of the grooves. Further rightward movement of the piston member 12 reduces the fraction of the length of the grooves 56 in communication with the chamber 26, and the fluid escaping from the closed chamber has to flow through a larger portion of the grooves. The resistance to flow afforded by the grooves thus progressively increases with further rightward movement of the piston, increasing the pressure force opposing such movement.

In order to control the mean relative positions of the piston member and cylinder there are provided valve means to vent fluid from the second chamber to the tank 40, and means to establish a limited communication between the first and second chambers. These valve means are constituted by the port 37 in the wall of the cylinder 10. When leftward movement of the piston member 12 (i.e. relative movement in the opposite sense) occurs, and a second predetermined position is reached, the grooves 56 overlap the port 37 and permit a limited flow of pressure fluid to pass from the second chamber 26 to the tank 40.

The limited communication between the chambers is established by a vent means comprising a duct 60 containing a valve 62 to transfer fluid from the first chamber to the second chamber. The valve 62 also is arranged to control the transducer. When in a first or open" position, this valve 62 allows relatively unlimited flow between the chambers 24, 26, and thus the pressures therein are at this time in phase and equal. Thus no oscillation of the piston member 12 occurs. When the valve 62 is set to a second or closed" position it is such that flow through the duct 60 is prevented except for a limited flow of fluid, associated with the venting of fluid from chamber 26. Oscillating movement of the piston member 12 thus occurs. The valve 62 may be set to intermediate positions whereby control of the amplitude of the oscillations is effected.

The valve 62 is such that when in the closed" position the average flow of fluid therethrough per cycle (taken over several cycles of oscillation of the piston member) is 5 percent to 20 percent, preferably 5 percent to l0 percent of the swept volume of the first chamber. The swept volume is equal to the gross oscillating pressure fluid flow into or out of the chamber 24 in each cycle. The fluid, of course, returns to the tank 40 via the port 37, and thus serves to cool the transducer. It has been found that the above-mentioned fluid flow rate controls the mean relative piston and cylinder positions and also gives adequate cooling, surprisingly without seriously impairing the performance of the transducer. it will be appreciated however that other embodiments of the invention may require greater or lesser flow.

The mean relative positions of the piston and the cylinder member (and thus the mean relative positions of the piston and the drill bit 32) are controlled by the through-flow in the following way. When the transducer is stationary, the pressures in the chambers 24, 26 are substantially equal, due to the duct 60, and the valve 62. The areas of the faces 20 and 22 of the piston member are chosen so that there is a net leftward force on the piston member which is thus predisposed to creep to the left in the cylinder 10. This is achieved by making the area of face 22 slightly greater than that of face 20. Alternatively, if use is made of the reaction force on the piston member 12 that occurs when it strikes the drill bit 32 during operation of the transducer, the area of face 22 can be very slightly less than that of face 20. The piston member will then be predisposed to creep leftwards during operation.

it can be shown that the velocity at which the piston member 12 strikes the drill bit 32, and therefore the force on the piston member 12 increases with increasing initial spacing of the piston member and drill bit, until an optimum spacing and impact velocity is obtained. This increases the predisposed leftward creep of the piston member 12, and this creep must be controlled so that the initial spacing of the piston member and drill bit is optimized.

The leftward creep of the piston member is controlled by means of the grooves 56 and the port 37, since when the chamber 26 is vented to the tank 40, the pressure therein is reduced and the pressure in the chamber 24 pushes the piston member 12 to the right.

Most of the flow via the duct 60 occurs while the pressure in the chamber 26 is reduced due to it being vented to the tank 40, since the pressure difference between the chamber 24 and 26 is then greatest. If the area of the piston face 20 is only slightly greater than that of the face 22 there may be at times during the cycle, some reversal of the flow through the duct 60, but this is acceptable provided the net flow (e.g. 5 percent to 10 percent) is maintained. If the face 20 is greater than the face 22, then to provide a leftward drift of the piston, it is desirable to introduce a nonreturn characteristic into the valve 62 when it is in its closed position, (e.g. by employing a stop valve in parallel with a nonretum valve as the valve lnanln a...

62) so that it permits greater flow from the chamber 24 to the chamber 26 than in the opposite direction. The flow in the duct 60 then occurs at times when the pressure in the chamber 24 is greater than that in the chamber 26.

It will be appreciated that the extent to which the grooves 56 overlap the port 37 is dependent upon the degree of leftward movement of the piston member 12. Thus the rate at which the chamber 26 is vented to the tank 40 is dependent on the fraction of the grooves 56 which is in communication with the tank 40 by overlapping the port 37. Thus, the greater the leftward movement of the piston in the cylinder, the more rapidly is the chamber 26 vented. The position of the port 37 and the length of the grooves 56 are of course chosen to ensure that the limit of leftward movement of the piston member 12 results in a gap between the mean position of the piston member and the drill bit 32 consistent with optimum efficiency of the transducer.

The position of the piston member 12 is thus accurately controlled enabling the mean position of the piston member to be accurately spaced from the drill bit 32.

Should a sudden drop in pressure occur in the chamber 24 e.g. due to fracture of the supply line 30, the piston may be urged rapidly leftward beyond its usual limit of travel. The grooves 56 then communicate with the chamber 38, venting the chamber 26 directly to atmosphere. This serves to reduce the leftward velocity of the piston member, and may prevent the piston striking the end wall 51 of the chamber 24.

FIG. 2 shows an alternative embodiment of the invention which is generally similar to that of FIG. 1 and which will not therefore be described in detail, the features thereof already described with reference to FIG. 1 being given the same reference numerals.

The portion 14 of the piston member 12 is not provided with axially extending grooves; instead, communication between the snubbing space 54 and the chamber 26 is afforded by at least one (preferably four) narrow radially extending conduits 57. Upon rightward movement of the piston member 12, the right hand end of piston portion 14 will enter the space 54, as in the FIG. I embodiment. Further rightward movement of the piston member 12 can only occur if some of the fluid in the closed space 54 is forced out via conduits 57.

Excessive leftward movement of the piston is controlled by a drilling 45 in the cylinder bore 36, which replaces the annular port 37 of FIG. I. When the piston member 12 moves leftward to beyond a predetermined position, the right-hand end of portion 14 uncovers the drilling 45, venting the chamber 26 to the relatively low pressure tank 40.

The single drilling 45 is employed in this embodiment in preference to the annular port 37 since the latter would produce too sudden a venting flow when uncovered by the portion 14 and the leftward movement of the piston member could not be accurately controlled. However, the drilling 45 is such that the transducer is again cooled as described earlier, the magnitude of the cooling flow being the same as in the FIG. 1 embodiment. Alternatively, an annular port 37 could be used if a restrictor is provided in the line 42 to cut down the flow rate.

It will be appreciated that, in either of the embodiments described, the piston member 12 could consist of two relative ly movable parts, the portion 16 being separate from the portions 14 and 18. In operation, such a two part piston would act as a one-piece piston member, but the parts thereof would be capable of relative movement to accommodate mal-alignmerits in the cylinder bores. The term piston member" as used throughout this specification is to be taken to include such a two-part piston.

It will be further appreciated that the pressure transducer may be made double-acting by connecting to the chamber 26 a source of oscillating pressure fluid which is 180 out of phase, i.e. in antiphase, with the source 28 connected to chamber 24. Such a double-acting transducer may produce up to twice as much power as the single-acting types hereinbefore described, but requires a double-acting source of oscillating pressure fluid and two connecting lines 30. In order to provide the coolingflow between the chambers the transducer is then arranged to be such that a flow occurs from the first chamber 24, and through the second chamber 26, either by employing a nonreturn valve between the first and second chambers or by arranging the areas of the piston faces such that the mean pressure in the second chamber is less than in the first chamber.

In either the single or double acting embodiments of the transducers the supply line 30 leading to the first chamber is replenished with cool pressure fluid from a point near the generator (not shown). In the double-acting embodiments, the flow, instead of being vented to a low pressure region via a port in the cylinder wall, may be vented from a point in the supply line 30 extending from the generator to the second chamber 26. This point is preferably near to the generator, so that the line is itself cooled.

It will be appreciated that the features of FIG. 2 may be applied singly to the transducer of FIG. 1 if so desired.

We claim:

1. -A transducer for converting fluid pressure oscillations into mechanical oscillations, comprising:

a cylinder;

a piston member in said cylinder;

said piston member and said cylinder being adapted for relative movement;

said cylinder having first and second chambers adapted to contain fluid under pressure;

said piston member having first and second opposing faces disposed in said first and second chambers respectively;

first means adapted to connect said first chamber to a source of oscillating pressure fluid to cause relative movement between said piston member and said cylinder in a first relative direction; second means associated with the second chamber to cause relative movement between said piston member and said cylinder in a second relative direction opposite to the first relative direction whereby said first and second means cause relative oscillating movement between said piston member and said cylinder;

said cylinder including means in said second chamber defining a snubbing space adapted to contain fluid;

said piston member including a portion adapted to enter said snubbing space and to compress fluid in said snubbing space upon excessive relative movement of said piston member and cylinder in one sense;

means defining a restricted outlet from said snubbing space when said portion of said piston member has entered said snubbing space;

means predisposing the piston member to creep in the second direction relative to the cylinder towards a predetermined position;

said last mentioned means including means adapted to permit limited flow of fluid from said first chamber to said second chamber; and

valve means operable upon relative movement of said piston member and cylinder to said predetermined position, to allow flow of fluid to a low pressure region from said second chamber.

2. A transducer as claimed in claim 1 in which the restricted outlet comprises at least one narrow conduit extending from the snubbing space to the second chamber.

3. A transducer as claimed in claim 2 in which the or each narrow conduit is a narrow groove.

4. A transducer asclaimed in claim 2 in which, upon initial said excessive relative movement, the at least one groove communicates with the snubbing space or the second chamber over a substantial fraction of its length, further excessive relative movement in the said one sense reducing the said fraction whereby the resistance afforded by the at least one groove to the flow of fluid therethrough is increased.

5. A transducer as claimed in claim 3 in which the piston member has a circumferential surface upon which the at least one groove is situated, the groove extending from the said second opposing face of the piston member towards the first opposing face for part of the axial length of the circumferential surface.

6. A transducer as claimed in claim 1 in which the average magnitude of the said limited flow of fluid per cycle of oscillation of the piston member is 5 percent to 20 percent of the swept volume of the first chamber.

7. A transducer as claimed in claim 6 wherein the said average magnitude is 5 percent to 10 percent of the said swept volume.

8. A transducer as claimed in claim 3 in which the valve means comprising a port in the wall of the cylinder, means connecting said port to the said low pressure region, the piston member having a circumferential surface upon which the at least one groove is situated, the groove extending from the said second opposing face of the piston member towards the first opposing face for part of the axial length of the circumferential surface, said at least one groove connecting said port to the second chamber upon said relative movement to said predetermined position.

9. A transducer as claimed in claim 8 in which when the piston member and the cylinder are in the said predetermined position, the at least one groove only communicates with the said port at its end remote from the said second face, further relative movement in the second direction causing the at least one groove to communicate with the said port at a part of its length nearer the said second face, reducing the resistance afforded by the at least one groove to the flow of fluid from the second chamber.

10. A transducer as claimed in claim 1 in which the means to permit limited flow comprises a duct containing a valve which when in a first position, allows a relatively unlimited flow of fluid to pass from the first chamber to the second chamber, and when in the second position allows said limited flow from the first chamber to the second chamber.

11. Apparatus comprising a source of oscillating pressure fluid and a transducer as claimed in claim 1 the source of oscillating pressure fluid being connected to the said first chamber and a driven load being connected to the piston member.

12. Apparatus as claimed in claim 11 comprising a further source of oscillating pressure fluid connected to the second chamber, the further source of pressure fluid being in antiphase relationship with the first-mentioned source. 

1. A transducer for converting fluid pressure oscillations into mechanical oscillations, comprising: a cylinder; a piston member in said cylinder; said piston member and said cylinder being adapted for relative movement; said cylinder having first and second chambers adapted to contain fluid under pressuRe; said piston member having first and second opposing faces disposed in said first and second chambers respectively; first means adapted to connect said first chamber to a source of oscillating pressure fluid to cause relative movement between said piston member and said cylinder in a first relative direction; second means associated with the second chamber to cause relative movement between said piston member and said cylinder in a second relative direction opposite to the first relative direction whereby said first and second means cause relative oscillating movement between said piston member and said cylinder; said cylinder including means in said second chamber defining a snubbing space adapted to contain fluid; said piston member including a portion adapted to enter said snubbing space and to compress fluid in said snubbing space upon excessive relative movement of said piston member and cylinder in one sense; means defining a restricted outlet from said snubbing space when said portion of said piston member has entered said snubbing space; means predisposing the piston member to creep in the second direction relative to the cylinder towards a predetermined position; said last mentioned means including means adapted to permit limited flow of fluid from said first chamber to said second chamber; and valve means operable upon relative movement of said piston member and cylinder to said predetermined position, to allow flow of fluid to a low pressure region from said second chamber.
 2. A transducer as claimed in claim 1 in which the restricted outlet comprises at least one narrow conduit extending from the snubbing space to the second chamber.
 3. A transducer as claimed in claim 2 in which the or each narrow conduit is a narrow groove.
 4. A transducer as claimed in claim 2 in which, upon initial said excessive relative movement, the at least one groove communicates with the snubbing space or the second chamber over a substantial fraction of its length, further excessive relative movement in the said one sense reducing the said fraction whereby the resistance afforded by the at least one groove to the flow of fluid therethrough is increased.
 5. A transducer as claimed in claim 3 in which the piston member has a circumferential surface upon which the at least one groove is situated, the groove extending from the said second opposing face of the piston member towards the first opposing face for part of the axial length of the circumferential surface.
 6. A transducer as claimed in claim 1 in which the average magnitude of the said limited flow of fluid per cycle of oscillation of the piston member is 5 percent to 20 percent of the swept volume of the first chamber.
 7. A transducer as claimed in claim 6 wherein the said average magnitude is 5 percent to 10 percent of the said swept volume.
 8. A transducer as claimed in claim 3 in which the valve means comprising a port in the wall of the cylinder, means connecting said port to the said low pressure region, the piston member having a circumferential surface upon which the at least one groove is situated, the groove extending from the said second opposing face of the piston member towards the first opposing face for part of the axial length of the circumferential surface, said at least one groove connecting said port to the second chamber upon said relative movement to said predetermined position.
 9. A transducer as claimed in claim 8 in which when the piston member and the cylinder are in the said predetermined position, the at least one groove only communicates with the said port at its end remote from the said second face, further relative movement in the second direction causing the at least one groove to communicate with the said port at a part of its length nearer the said second face, reducing the resistance afforded by the at least one groove to the flow of fluid from the second chamber.
 10. A transducer as claimed iN claim 1 in which the means to permit limited flow comprises a duct containing a valve which when in a first position, allows a relatively unlimited flow of fluid to pass from the first chamber to the second chamber, and when in the second position allows said limited flow from the first chamber to the second chamber.
 11. Apparatus comprising a source of oscillating pressure fluid and a transducer as claimed in claim 1 the source of oscillating pressure fluid being connected to the said first chamber and a driven load being connected to the piston member.
 12. Apparatus as claimed in claim 11 comprising a further source of oscillating pressure fluid connected to the second chamber, the further source of pressure fluid being in antiphase relationship with the first-mentioned source. 